FAN5308_07 FAIRCHILD [Fairchild Semiconductor], FAN5308_07 Datasheet - Page 9

• Manufacturer Part Number: FAN5308_07
• Description: 800mA High-Efficiency Step-Down DC-DC Converter
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

©2005 Fairchild Semiconductor Corporation
FAN5308 Rev. 1.0.2
Applications Information
Setting the Output Voltage
The internal reference is 0.8V (typical). The output volt-
age is divided by a resistor divider, R1 and R2 to the FB
pin. The output voltage is given by:
where R
According to this equation, and assuming desired output
voltage of 1.5096V, and given R2 = 10k , the calculated
value of R1 is 8.87k . If quiescent current is a key
design parameter, a higher value feedback resistor can
be used (e.g. R2 = 100k ) and a small bypass capacitor
of 10pF is required in parallel with the upper resistor, as
shown in Figure 16.
Inductor Selection
The inductor parameters directly related to the device’s
performances are saturation current and DC resistance.
The FAN5308 operates with a typical inductor value of
3.3µH. The lower the DC resistance, the higher the effi-
ciency. For saturation current, the inductor should be
rated higher than the maximum load current plus half of
the inductor ripple current.
This is calculated as follows:
where:
f = Switching Frequency
L = Inductor Value
10 F
V
I
C
L
I
OUT
Inductor Value
L
IN
= Inductor Ripple Current
=
V
1
=
3.3µH
3.3µH
PGND
Figure 16. Setting the Output Voltage
+ R
OUT
V
Table 1: Recommended Inductors
V
EN
IN
REF
2
< 800K .
1
2
3
1
----------------------------------------------- -
(AGND)
–
P1
V
1
OUT
+
L
Panasonic
6
5
4
R1
------- -
R2
Vendor
f
Murata
SW
NC
FB
V
3.3 H
IN
R2
R1
10K
5K
C
LQS66C3R3M04
f
Part Number
ELL6PM3R3N
C
2 x 10 F
OUT
1.2V (800mA)
V
OUT
EQ. 3
EQ. 4
9
Capacitors Selection
For best performances, a low-ESR input capacitor is
required. A ceramic capacitor of at least 10µF, placed
close to the V
output capacitor determines the output ripple and the
transient response.
PCB Layout Recommendations
The recommended PCB layout is shown in Figure 17. The
inherently high peak currents and switching frequency of
power supplies require careful PCB layout design.
Use wide traces for high-current paths and place the
input capacitor, the inductor, and the output capacitor as
close as possible to the integrated circuit terminals. To
minimize voltage stress to the device resulting from ever-
present switching spikes, use an input bypass capacitor
with low ESR. Note that the peak amplitude of the
switching spikes depends upon the load current; the
higher the load current, the higher the switching spikes.
The resistor divider that sets the output voltage should
be routed away from the inductor to avoid RF coupling.
The ground plane at the bottom side of the PCB acts as
an electromagnetic shield to reduce EMI.
For more board layout recommendations, download the
Fairchild application note PCB Grounding System and
FAN2001/FAN2011 High-Performance DC-DC Converters
(AN-42036).
Capacitor
Value
10µF
Figure 17. Recommended PCB Layout
Table 2: Recommended Capacitors
IN
and AGND pins, is recommended. The
Vendor
Murata
Yuden
Taiyo
TDK
GRM32ER61C106K
C3216X5ROJ106M
C2012X5ROJ106K
JMK212BJ106MG
JMK316BJ106KL
Part Number
www.fairchildsemi.com